It is becoming more and more common that optical stereo devices, such as stereo cameras, are being used in a number of different fields. For instance, in the medical field stereo cameras and/or stereo microscopes are increasingly employed for assisting medical professionals in performing such tasks as medical surgeries, examinations and the like. This is because stereo cameras or microscopes can provide a three dimensional view of and, thus, further information about an object of interest. Typically, optical stereo devices provide for the possibility of zooming into a picture by means of a special lens system, such as a pancratic lens system. Some sophisticated stereo microscopes or stereo cameras include an autofocus mechanism, i.e. the ability to automatically bring the object of interest into focus. Such an autofocus mechanism can be provided, for instance, by means of a control device and operatively connected driving means that focus the lens system of the stereo microscope or stereo camera by moving the lens system to the best or optimal focus position, i.e. the position where the sharpest possible image of the object of interest is obtained.
Conventionally, the best focus position of a stereo microscope or stereo camera can be obtained automatically by several known methods. For instance, a first known autofocus method not restricted to stereo microscopes or stereo cameras is based on the fact that the contrast or sharpness of the object of interest generally is at maximum at the best focus position of the camera or microscope. The contrast of the image of the object of interest is generally determined by means of an image processing software running on a control unit and the focal length and/or distance between the camera or microscope and the object of interest is changed iteratively in the direction of higher contrast until no further improvement of the image quality is discernible. However, this known autofocus method, i.e. determining the best focus position by means of a contrast optimization, has the disadvantage that in most cases initially one has no information about (i) how far the current focus position is off the best or optimal focus position and (ii) in which direction the best focus position is with respect to the current focus position. Moreover, the relationship between the image quality and the distance of the actual focus position to the optimal focus position is actually not linear, but rather exponential. In other words, close to the optimal focus position the image quality changes much more rapidly than further away from the optimal focus position. As a consequence thereof, it is generally very difficult to find a range of acceptable focus positions around the optimal focus position as a first “guess”, because outside of such a range of acceptable focus positions the image quality changes only gradually. Thus, autofocus methods based on contrast maximization are computationally quite challenging and because of the iterative approach generally not very fast.
Other known autofocus methods for optical stereo devices measure the reflection of a laser beam within the focus plane or use a specific spatial arrangement of two laser beams that coincide in the focus plane.
The object of the present invention is to provide an optical stereo device, such as a stereo camera or stereo microscope, that is configured to automatically focus its imaging unit in a new and improved way as well as a corresponding new and improved autofocus method.